Since small private aircraft have come into popular use, thefts of such craft have become a serious problem. It is practically impossible, due to the type of ignition and other features of construction of small aircraft, to prevent unauthorized individuals from removing and replacing wires and then operating the aircraft. To solve such problem it has been proposed to apply a mass of predetermined weight to the aircraft propeller in a manner to unbalance it. If the engine is started with the propeller unbalanced severe vibration will result and the craft cannot be flown.